An X-ray tube is a type of vacuum tube that converts electrical power into X-rays. In such devices, the cathode is a negatively charged electron emitter and the anode is a positively charged electron collector. Between the cathode and the anode, a flow of electrical current is established using a high voltage power source, typically between 4 kV and 500 kV, which accelerates the electrons in their path. However, efficiency of the X-ray tube is very low and the yield of X-ray production is usually less than 1%, with the remaining 99% of the input energy being converted into heat.
The excess heat that is produced in the X-ray tube must be removed from the high voltage anode in order to prevent the device from overheating and to enable continuous operation. Thus, thermal management of this generated heat is an important consideration in the design and manufacture of X-ray tubes in addition to the electrical insulation considerations. For example, the high voltage anode must be electrically isolated from mechanical components of the X-ray tube including the mechanical support, and at the same time be capable of removing excess heat.
In the past, in order to avoid overheating, X-ray tubes have been immersed in cooling fluids such as oil that are capable of heat transfer through a process of heat convection and which have high dielectric strength so that heat is dissipated away from high voltage anode as the cooling fluid circulates. More recently, it has been proposed to use anode stacks that sit between the high voltage anode and the mechanical support and which are made from materials that are both electrically insulating and thermally conductive in order to achieve the same cooling effect but without the burden and constraints of a liquid coolant.
In some anode stack designs, a dielectric disc of high thermal conductivity is sandwiched between two metallic discs, also of high thermal conductivity, one of which is coupled to the high voltage anode, and the other of which is coupled to a heat sink. In these devices, the high thermal conductivity of the dielectric and metallic discs allows effective heat transfer away from the high voltage anode.
However, this design has a number of drawbacks related to high electric fields at the triple point region, i.e. the region where the dielectric, metal and vacuum meet; the electric field at the triple point regions of traditional anode stacks can be much greater than the surrounding electric field depending on the geometry of the component parts of the anode stack and the dielectric constants of the materials. These locally intensified electric fields are often the cause of electrical arcing and electrical breakdown that ultimately leads to device failure.